Ionic microanalyzers

ABSTRACT

In an ionic microanalyzer, the energy filtering of the ions of the beam providing an image of a sample is effected by means of a spherical capacitor, suitably associated with the magnetic deflector used for the momentum-to-charge ratio filtering, through an intermediate lens system.

United States Patent inventors Raymond Castaing;

Georges Slodzian, both of Paris, France Appl. No. 871,675 Filed Nov. 19,1969 Patented June 15, 1971 Assignee Centre National De La RechercheScientifique and Thomson-CSF Priority Nov. 28, 1962 France 916,836 Pat.1,352,167

Continuation of application Ser. No. 712,406, Mar. 12, 1968,Continuation-inpart of application Ser. No. 518,453, Jan. 3, 1966, nowabandoned Continuation of application Ser. No. 326,566, Nov. 27,

References Cited UNITED STATES PATENTS 2,947,868 8/1960 Herzog 250/4192,957,985 10/1960 Brubaker 250/419 3,061,720 10/1962 Ewald 250/4193,233,099 2/1966 Berry et a1. 250/419 FOREIGN PATENTS 1,240,658 8/1960France 250/419 OTHER REFERENCES Mass Spectrometer Image DisplacementsDue To Second- Order Aberrations by C. F. Robinson from The Review ofScientific Instruments, Vol. 29, No. 7, July, 1958, pages 622 624.

Primary ExaminerWilliam F. Lindquist Attorney-Cushman, Darby & CushmanABSTRACT: In anionic microanalyzer, the energy filtering of the ions ofthe beam providing an image of a sample is efiected by means of aspherical capacitor, suitably associated with the magnetic deflectorused for the momentum-to-charge ratio filtering, through an intermediatelens system.

M01. 745: sou/P05 PATENTEU JUN} 5 l9?! SHEET 3 OF 6 on c MICROANALYSERSThis application is a continuation-in-part of our application Ser. No.712,406, filed March 12, 1968, which was a continuation-in-part of ourapplication Ser. No. 518,453, filed Jan. 3, I966, (now abandoned) whichwas a continuation of our application Ser. No. 326,566, filed Nov. 27,I963 (now abandoned) for Improvements in microanalyzers by secondaryemission.

The present invention relates to the mass-resolving system ofmicroanalyzers making use of the secondary ion emission for producing,by means of a particle system which combines ion optics and massspectrometry, characteristic images" of the surface of the sample whichindicate the map of distribution of its various elements or isotopes.

Such microanalyzers are known in the art. A suitable system wasdescribed in the French Pat. No. 1,240,658.

The images must be selective with respect to the mass m of the secondaryions. In fact, as in mass spectrometry, the selectivity can be obtainedonly as a function of the ratio m/q, where q is the charge of the ion.This being understood, it will be assumed hereinafter that the ionsutilized for forming the images are positive ions, with a single charge;this will not be a restriction as to the use of the arrangementdescribed since an ion with a mass m and a charge 2: behaves in anelectric or a magnetic field as an ion with a mass m/2 and a charge q.

It is known that in an ionic microanalyzer, the secondary ions, obtainedby the impact of primary particles, are emitted from the sample surfacewith a certain amount of energy dispersion.

As in a mass spectrometer, these secondary ions are thereafteraccelerated by a fixed voltage V,,, which is applied between thespecimen and an accelerating electrode, and imparts to each of the ionsan energy E, equal to V electronvolts, the total energy of the ion beingthen V,,+AV, where AV is the initial energy, in electron volts, of theion.

In order to minimize the ratio AV/V,,, V, should be high. However, forpractical reasons, V cannot be chosen as high as it would be desirablein this respect, and the energy dispersion of the secondary ion beam isnever negligible.

Since the magnetic filtering of the ions is not selective with respectto the mass m of the ions, but to their momentum mv, where v is the ionvelocity (m v/q if all the ions have not the same charge) a satisfactorymass filtering of the ions requires, as is often the case in massspectrometry, the adjunction of a filtering with respect to energy, inaddition to the filtering with respect to momentum, so as to retain inthe useful beam only those ions whose initial energy is lower than agiven threshold, or lies in a predetermined energy band, the threshold,or the band, being a function of the mass of the ions used for the imageand of the masses of the other ions present in the secondary ion beam.

In a microanalyzer, this energy filtering raises however problems whichare not encountered in mass spectrometry and which are due to thefollowing factors:

In mass spectrometry, the energy filtering needs only to preserve theimage of a narrow slit, generally considered as a one-dimensionalaperture. In a microanalyzer strict conditions must be respected for twotwo-dimensional images, i.e. the image of a two-dimensional aperture,and the image of the sample surface.

One solution of this problem consists in effecting the energy filteringwith the help of an electrostatic mirror.

This system, however, can be used only for imparting a maximum threshold(and not a minimum one) to the energy of the ions, which energyfiltering is generally, but not always sufficient. On the other hand,where very high ion currents are used, the operation of the mirror maybe perturbed by space charge in the vicinity of the reflectingelectrode, where the velocity of the noneliminated ions is zero.

According to the present invention, which eliminates the above-mentioneddrawbacks, the energy filtering is effected by means of a sphericalcapacitor associated with auxiliary lens means, the spherical capacitorhaving optical properties which, in the problem considered here, cancompare with those of the mirror, and allowing a high degree ofachromaticity in the image finally obtained.

It is recalled here that in particle optics, the term achromaticity asapplied to an image is used to indicate the absence of aberrations dueto the fact that the particles used for forming the image have differentvelocities.

The invention will be better understood and other characteristicsthereof will appear, from the following description and appendeddrawings, in which:

FIG. 1 is a diagram illustrating the principle of the operation of amagnetic prism used in a known microanalyzer;

FIG. 2 illustrates the optical properties, used in the presentinvention, of a spherical capacitor;

FIGS. 3 to 6 are the optical diagrams of various preferred embodimentsof the double filtering system according to the invention;

FIG. 7 is a detailed embodiment of an ionic microanalyzer using theoptical diagram of FIG. 6.

The apparatus of the invention uses two deflectors having a commonradial plane. Each of those deflectors has a pair of conjugate realstigmatic-points for ions having a predetermined mass and velocity, andentering it along a given axis.

A convenient magnetic deflector of this type, already used in knownmicroanalyzers, is a magnetic prism. It is known that a magnetic prismor sector is a space region limited, along two faces, by a dihedron,with, disregarding edge effect, a uniform magnetic induction parallel tothe line of intersection of the planes of the dihedron, which line willbe assumed here to be vertical. It is generally obtained by means of anelectromagnet whose pole pieces, of adequate design, having parallelplane inner faces, separated by a gap the width of which is small ascompared with the dimension of the pole pieces. A radial plane of amagnetic deflector being a plane perpendicular to the magneticinduction, a magnetic prism has more than one radial plane. The radialplane more particularly considered here is the plane of symmetry of thetwo pole pieces.

FIG. I shows the section in the radial plane of a magnetic deflector l,which is a deflector, i.e. a deflector deflecting by an angle of 90 thecenter ray of the beam of particles which it is desired to select. Twoaxes ZZ and Z'Z',, contained in this plane and normal to each other,intersect each other at H and the two faces of the prism respectively atI and l, H I being equal to H I.

sand 6' are the oriented acute angles of the normals at I and Irespectively with the straight lines ZZ, and Z'Z,.

An ion P,, with a mass m,,, and a velocity v, directed along Zl,entering the prism at I has its trajectory inside this prism incurvedalong an arc of a circle, tangent at I to ZI, the radius R of thiscircle being proportional to m v,,/B where B is the magnetic inductioninside the prism.

B may be adjusted so that R=l H; the ion P then leaves the prism 'at I',and its trajectory follows the half-line I'Z,, tangent at I to the arcII.

This trajectory ZIIZ' is defined as the optical axis" of the prism for abundle of trajectories in the vicinity thereof. The surface normal tothe radial plane along the optical axis is called the transverse sectionof the prism.

It is convenient to indicate the optical properties of the prismconsidering only the rectilinear portions of the trajectories, outsidethe prism, and their virtual prolongations into the prism.

These trajectories will thus be related to the two axes ZI-IZ andZ'HZ',, which will be respectively designated as the object axis, andthe image axis. The origins of the ordinates on those two axes arerespectively I and I, and the positive directions thereof are from Z toZ, and Z' to Z 1 respectively.

The focusing properties of the prism for trajectories contained in theradial plane are well known and applied in mass spectrometers toconjugate the entrance slit and the exit slit optically.

ln particular, there exists, for the trajectories of the radial plane inthe vicinity of the optical axis, an object-side focal point F, animage-side focal point F and a focal distance f,,, such that any pointN, real or virtual, of the object axis has a conjugate image point N,real or virtual, of the image axis, N being such that The focusingproperties of the prism for trajectories lying in the transverse sectionalso find an application in a microanalyzer. As is known theseproperties, which are based on the existence of a small radialcomponent, due to an edge effect, of the magnetic induction in thevicinity of the prism edges, exist only if the optical axis of the prismis at an angle with the normal to the prism at the entrance or/and atthe emergence face thereof. The horizontal component, which is normal tothe prism face, is zero in the radial symmetry plane of the prism, andincreases, with opposite directions, above and under this plane, and, ifit is at an angle with the velocity vector of the ions, produces aforce, a component of which is vertical.

The trajectories lying in the transverse section and crossing underthese conditions a face of the prism are correspondingly incurved; thisresults in focusing properties of the prism in the transverse section.

However, the focal points and the focal distance for the transversesection are generally different from those pertaining to the radialplane.

More precisely, to any object point with an abscissa on the object axis,there corresponds on the image axis, a first image point, conjugate ofthe object point in the radial plane, with an abscissa and another imagepoint, conjugate of the object point in the transverse section, with anabscissa For reasons, which will be set forth hereinafter, it ispreferred to have a symmetrical design of the prism system, i.e. to havewe. ZZ, and Z'Z', are then symmetrical with respect to the verticalplane bisecting the prism.

Under those conditions, the formulas giving and given in the generalcase by the Cotte theory (Maurice Cotte, doctorate thesis Recherches surloptique electronique," Masson et Cie Paris, 1938), become (1 time)(-tane tine A beam of revolution with its apex in M and its axis along21 will thus become after the crossing of the prism a beam substantiallyconverging in M and which, by reason of symmetry, will be a beam ofrevolution about lZ',. The advantage of making e e is thus clearly seen,since it avoids aberrations of the images through preserving thesymmetry of revolution of the beam.

On the other hand, among all the couples of points N and N, conjugate inthe radial plane, and respectively lying on the object and the imageaxis, there exists a couple of points C and C such that a plane beam ofthe radial plane, having its apex in C and its axis along Cl isconverted by the prism into a beam converging in C, not only for ionsP,,, but also, disregarding first order terms, for all those ions whosemomentum is slightly different from m v in other words, as concerns ionswith the mass m,,, for a certain velocity band including v Point C willbe called hereinafter the achromatic focal point of the prism. For theparticular prism considered above, C and C are respectively given by lnthe absence of an energy filtering, the microanalyzer of the known typeoperates as follows (FIG. 1):

A magnetic prism of the above type is used.

The surface of the specimen 5 to be analyzed is placed normally to theaxis Zl.

A plurality of electrodes 6, forming with the specimen surface animmersion lens, through suitable voltages being applied to the sampleand to the electrodes, accelerates the ions and imparts thus to them asupplemental energy E,,=% m, V, =V,, electron-volts, and is so locatedthat its crossover Q is centered on M. The convergence of the lens is soadjusted that the secondary ions extracted from the specimen under theimpact of primary particles, for example primary ions, are concentratedinto a beam giving a real magnified image S, of the analyzed samplesurface (i.e. a small portion of the surface of the specimen), whichimage, in the absence of the prism would be centered on point C.

Under those conditions, the prism gives in C an image S, of S,, andconsequently ofthe analyzed surface, and, in M, an image Q, of thecrossover Q, the latter being stigmatic, while the former, which isconjugate of S, only as concerns the radial focusing, is astigmatic.

Two diaphragms 3 and 13, with circular apertures, are respectivelylocated in M and M. These diaphragms, as concerns their selectivity inthe radial plane, play the part of the selection slits of a conventionalmass spectrometer and ensure the momentum filtering of the secondary ionbeam.

The vertical image S,, practically achromatic for ions with mass m and avelocity comprised in a velocity band including v is converted into areal image by means of a lens 7. However, this image, as indicatedabove, may be contaminated by ions having a mass different from m,,.

On the other hand, the image 8, is astigmatic in the transversedirection, but this is a defect which it is easy to correct on the finalimage by means of a conventional stigmator 8 placed in the vicinity ofthe diaphragm 13, so that it will not substantially perturb thestigmatism obtained for 0,.

FIG. 2 shows the operation, considered per se, of a spherical capacitorused in particle optics.

It will be recalled that, in the case of an electric field, and of aplane optical axis, the radial plane is that plane which contains boththe field and the axis, in other words the unique plane containing theoptical axis, where the latter is incurved.

in H6. 2 a spherical capacitor, with two electrodes P, and P has beenshown by its section in a diametral plane, assumed to be horizontal,this capacitor being limited by two vertical planes wX and wY. An arc Kof a circle, having the same center to as the electrodes P, and P and aradius r which is the half-sum of the radii ofP, and P has also beenshown.

Suitable voltages are applied to electrodes P, and P to make thepotential along K equal to the potential outside of the capacitor, thepotential difference between P and P, being such that ions with a massm, penetrating into the capacitor in i, intersection of the arc K withwX, with a velocity v directed along the tangent zi to the arc K,describe this arc K inside the capacitor and consequently leave it alongthe tangent iz, to the arc K, i being the intersection of this e wit QY-i The system with the optical axis ziiz, has in a radial plane opticalproperties similar to those of a magnetic prism in a radial plane, andwhich will be set forth in the same way, using the virtual prolongationsinto the capacitor of the rays penetrating therein, as well as theoriented axes ziz and ziz' which are the object axis and the image axisof the system.

In this system, an object-side focal point f and an image-side focalpoint I may be respectively determined on the'object axis and the imageaxis, and the optical properties indicated for the radial plane of themagnetic prism hold for the capacitor, iff,f', zz zz, are substitutedfor F, F, 22 and ZZ',, and a constant f for the constant f,,.

But the spherical capacitor has in addition the property that what hasbeen said for the radial focusing holds for the focusing in thetransverse section, i.e. the vertical surface cutting the radial planealong the optical axis, with the same focal points and the same focallength f,. It is to be noted that this latter property is specific ofspherical capacitors and does not belong to the cylindrical capacitorsgenerally used in mass spectrometry.

Lastly the Applicants have established that two conjugate points c and 0may be defined, for which the direction focusing, as well the radial asthe transverse one, is substantially independent of the energy of theions. There is thus obtained a focusing for an energy band centered onm, v lo, and consequently, as concerns the ions with the mass m for acertain range of velocities centered on v,,. Point 0 will be calledhereinafter the achromatic focal point of the capacitor.

It is thus possible, with a spherical capacitor, to obtain an achromaticenergy filtered ionic image, in the same way as an achromatic momentumfiltered image may be obtained with a magnetic prism. It sufiices tochose two conjugate real points In and m, points m, m, c and c playingthen the part previously played by points M, M, C and C. But thestigmator is here necessary only to correct the edge effect in thecapacitor.

The two deflecting sectors, i.e. the prism and capacitor, are seriallylocated, preferably so that the first sector (prism or capacitor)supplies an achromatic filtered image of the sample, which image istaken up by a conventional optical system to be projected, with asuitable magnification, onto the point which is the conjugate, at leastin the radial plane, of the achromatic focal point of the second sector,the latter supplying thus an achromatic double filtered image.

Of course, the vertical astigmatism correction must be effected on theimage supplied by the magnetic sector.

There is thus obtained a purely mass-filtered image, which is achromaticfor a velocity band including v This image is again taken up by aconventional optical system to be projected onto an observation screen,preferably through an ion-electron image converter.

Moreover, the invention has for its object more particular associationsof the two deflecting sectors, such that ions with a mass m penetratinginto the first sector with a velocity vector directed along the objectaxis of this sector leave the second sector along the image axisthereof, independently, disregarding terms of the second order, of thevalue of this velocity.

There is thus obtained an achromatic system allowing a still largercontribution of the ions with the mass m, to the final image, whilestill avoiding-the intrusion, in the final image, of ions with adifferent mass. In addition, the system is achromatic not only for theions of mass m but also for the ions whose mass is slightly differentfrom m,,, which ensures a very high resolving power of the apparatus.

Before describing associations of the two sectors, the electric sectorwhich is preferably used will be more accurately determined. Again here,it is preferred to use, as shown in FIG. 2, an optical axis whosecircular portion is a 90 arc, for reasons of convenience, and alsobecause it facilitates the preferred association modes of the twosectors.

In order to obtain this 90 arc, it obviously suffices that (FIG. 2) taxand wY should be perpendicular to each other, the two focal points f andf then coincide with i and 1'' respectively, and the conjugationrelation is The achromatic focal point e (which is accurate disregarding second order terms) coincides with the point of intersection of theobject and image axes, as well as with its conjugate c.

FIGS. 3 to 6 show several embodiments of the association of thepreferred type. Onthose Figures, the same elements are designated by thesame symbols as in FIGS. 1 and 2.

On the other hand, the plane of all the Figures is the symmetry radialplane of the magnetic sector and a radial plane of the electric sector;the object axis of the second sector coincides with the image axis ofthe first one, those two superimposed axes being hereinafter designatedas the common axis. This being so, either of the two sectors may occupya position derived from thatof FIGS. 1 or 2, not only through adisplacement in which the radial plane slides on itself, but alsothrough a rotation around its object or image axis, which of course doesnot alter its properties in the least way.

According to whether the two optical axes lie or not on one and the sameside of the common axis, the assembly will be called an assembly of theC type or of the S type.

Lastly, for all the described assemblies, the term angle of dispersionwill mean that angle which is formed between the image axis of thesecond sector and the emergence rectilinear trajectory of an ion of massm +Am and energy V +A V, having penetrated into the first sector with avelocity vector directed along the object axis of the first sector.

In FIG. 3, an assembly of the C type has been shown, wherein the firstsector is the magnetic prism. The prism of FIG. 1 and all the elementspreceding it are again present in the assembly of FIG. 3.

But the lens 7 is no longer there and is substituted by a lens 9, whichis, like the stigmator 8, located in the vicinity of point M. Thediaphragm 13 is, as previously, centered on point M. The lens 9 is verynear M so that it is possible, for the sake of simplicity in thelanguage and formulas, to consider that the image which it gives ofpoint M practically coincides with M.

Point i of the electric sector is at a distance D from M. Image point Mof the prism optical system may also be considered as an object point mof the capacitor optical system, and is thus called point M, m in theFigures. The convergence of the lens 9 is adjusted to give of thevirtual image 8' given by the prism, a real image s, centered on pointc, c of the spherical capacitor, the latter image playing the part of avirtual object for the capacitor, which gives thereof a virtual images',, also centered on point c,c, but normal to its image axis. On theother hand, the capacitor gives of the crossover in M,m a further imagecentered on point m, conjugate of point M, m in the capacitor opticalsystem, i.e. defined by im=r (-D), (D being a positive length).

At this point m, a further diaphragm 23 is located. The lens 18, whichforms the ultimate ion image is centered on this object axis, beyond thediaphragm 23.

Such a structure allows the obtention of an achromatic system:Calculation shows that the angle of dispersion or of the system is Itsuffices to make Dl'FZ R to have a system, which is dispersive withrespect to mass, without being dispersive with respect to energy orvelocity, since in that case In particular, as shown in FIG. 3, thisrelation obtains for D=rR.

Under those conditions, the velocity pass-band" of the ions of mass m,is optimum.

FIG. 4 shows an assembly of the C type, wherein the capacitor precedesthe prism.

Point m lies on the half-axis z i. The specimen and an accelerating lens16 are placed normally to the axis z z, and the lens adjusted so as togive a crossover Q centered on m and the real image s. of the samplesurface. This real image acts as a virtual object for the capacitorwhich gives thereof a virtual image s".

A diaphragm 33 is centered on m to delimitate the crossover The magneticsector is so located that the point M of its optical axis coincides withthe point m, which is the conjugate of point m in the electric s;e c tor and whose position on the object axis thereof is given by i'm'=r/E=D.

At point m,M, as at point M',m of FIG. 3, a diaphragm 43 is located,and, in the immediate vicinity thereof, a lens 9, which allows theprojection, onto point C of the prism, of a real image S, of the virtualimage r, given by the capacitor; the image 8, is for the prism a virtualobject.

The conditions are now again those which were described, with the helpof FIG. 1, for the obtention of an image filtered only as to themomentum of the ions. The corresponding ele ments have not been shownagain in FIG. 4.

The angle of dispersion of the assembly of FIG. 4 is An achromaticsystem is thus obtained for The mass dispersion being then In particularthis condition obtains. as shown in FIG. 4, for

It has been assumed, hereinabove, that the optical center of the lens 9(FIGS. 3 and 4) practically coincided with the center of the diaphragm,13 or 43, respectively. In fact, this condition is not easily met with avery good degree of accuracy, because the lens is generally anelectrostatic one, and it is difficult to place a diaphragm very nearits center without perturbing its operation.

Besides, the lens considered here has a comparatively large focallength, and its dimensions are large.

It is thus preferred to substitute for the single lens centered on thecommon axis a group of two lenses, each of which has then a smallerfocal length than that of the single lens.

As will be shown, it is thus possible to realize assemblies of the S"type, which from the point of view of dispersion and achromatism, haveproperties respectively similar to those of the C type assemblies with asingle intermediate lens, this being due to the fact that the passagefrom the C type to the 8" type is associated with the change of aninverted image to a noninverted image as concerns the sample image givenby the intermediate lens system.

FIG. 5 shows an arrangement of the "S" type where the prism precedes thecapacitor. The structure is the same as that of FIG. 3 between thesample and point M.

The diaphragm l3 delimiting the first image of the initial crossover iscentered on M.

A first convergent lens 31, with a focal length f,, has its object-sidefocal point in M and a second convergent lens 32, with a focal lengthf,, is placed, relatively to the capacitor, so that its image-side focalpoint coincides with point In of the capacitor optical system. There isthus obtained in m an image ofthe crossover in M.

The distance L between the two lenses is determined so that the pointsC' of the prism optical system and c ofthe capacitor optical system areconjugate with respect to the two-lens system.

The conditions are now the same, for obtaining the final image, as inthe case of FIG. 3.

The angle ofdispersion of this system is An achromatic system isobtained for If f,=f an achromatic system is obtained for FIG. 6 is anarrangement of the S" type, with two intermediate lenses, wherein theelectric sector precedes the magnetic sector.

Nothing is changed, relatively to FIG. 4, between the sample and pointm.

The intermediate optical system may be the same as that of FIG. 5, thediaphragm 43 being centered on m, where the object-side focal point oflens 31 is also located, while the imageside focal point of the lens 32coincides with the point M of the prism optical system, and the distanceL between the two lenses being such that the point c and C are conjugatewith respect to the two-lens system.

The conditions are now the same as those which prevailed in the case ofFIG. 4 for the obtention of the final image.

The angle of dispersion is With f =f the condition D+r=2 R is againobtained.

The apparatus shown in FIG. 7 is a practical embodiment of amicroanalyzer according to the optical diagram of FIG. 6. It comprisesfive main parts connected so as to build up a vacuumtight enclosure inwhich a vacuum is maintained with the help of a pumping system not shownin the Figure.

The first part is the specimen chamber 51 wherein the specimen 5 to beanalyzed is placed on a support 53, which may be displaced along threedirections, each of which is perpendicular to the other two, by means ofa mechanism 54, the controls of which are outside the specimen chamber.

An ion gun 55, fixed in the specimen chamber generates the beam ofprimary ions for the bombarding of the sample. The secondary ions, whichwill be assumed to be positive ions, are accelerated and focused bymeans of a three-electrode lens 16 forming with the sample an immersionlens. To this end, the sample is brought, by means of a voltage source57, to a potential V which is positive relatively to the groundpotential of the apparatus. Three potentiometers 61, 62 and 63 areconnected across this voltage source; the first and third electrodes ofthe lens 16 are grounded, while the second electrode is brought by meansof the potentiometer 61 to an adjustable positive potential less than V,in order to adjust the convergence of the lens. A diaphragm 33 is placedat the crossover of this lens.

The second part of the apparatus is an enclosure 59 comprising theelectric sector, built up by the spherical electrodes P, and Prespectively brought to potentials V and +V means of a voltage source162.

The third part is the intermediate enclosure 163 containing the energyselection diaphragm 43, and the two three-electrode lenses 31 and 32,whose convergences are adjusted. by means of the potentiometers 62 and63, whose sliders are connected to the second electrode of those twolenses respectively. I

The fourth part is the enclosure 67, containing the pole pieces thelower one of which, 68, is visible in the Figure, of an electromagnet l9generating the magnetic prism. The coils 70 of this electromagnet arelocated outside the enclosure 67 and connected to an adjustable currentsource 71 for the adjustment of the magnetic induction.

The fifth part is the enclosure 72 containing the stigmator 8, thediaphragm 13 and the image converter, the latter and the electric supplythereof not being shown in the Figure. A binocular-viewing device 75 isused to observe the image ap pearing on the luminescent screen of theimage converter.

Of course, the invention is not limited to the embodiment shown anddescribed.

In particular, it is possible to use for the electric sector as well asfor the magnetic sector optical axes whose circular portions are builtup by arcs other than 90 arcs, it being always possible to determine onthe former as well as on the latter an achromatic focal point.

Also, it is of course possible to use a "C" type assembly with atwo-lens intermediate system, or an S-type assembly with a one-lensintermediate system; however, the degree of achromaticity thus obtainedis not so satisfactory as in the described embodiments.

It should be noted that the stigmator which is necessary in thedescribed embodiments to correct the image of the sample surface givenby the prism is not, in fact, a drawback of the embodiments described,since anyhow a stigmator is always necessary in'such complex opticalsystems to correct minor astigmatisms such as those resulting from edgeeffects, and since, generally, the different astigmatisms may becorrected by means of a suitably designed single stigmator.

In this connection, it should be noted that the stigmator should be, forthe reason hereinabove indicated, located in the vicinity of one of thecrossovers.

We claim:

1. An ionic microanalyzer for providing a selective ionic image of asurface of a sample, said microanalyzer comprising:

means for bombarding a surface of a sample with primary particles,thereby extracting secondary ions from said surface;

first lens means, having an axis, for accelerating said ions andconcentrating them into a beam having a crossover centered at apredetermined point of said axis of said lens means and providing afirst image of said surface, said first image being centered at afurther point of said axis;

means for filtering the ions of said beam with respect to theirmass-to-charge ratio, said filtering means comprism a first and a seconddeflector, one of which is a magnetic deflector having two pole piecesdefining a gap having lateral faces normal to said pole pieces, and theother of which is a spherical capacitor,

said deflectors having a common-radial plane with respect to which saidtwo pole pieces are symmetrical with each other,

each of said deflectors having in said radial plane an optical axiscomprising, inside the considered deflector, an arc of a circle havingtwo ends, which are is preceded and followed respectively by a first anda second rectilinear portion, respectively tangent to said are at saidtwo ends thereof, and respectively lying on a first and secondrectilinear axis, respectively referred to as the object axis and theimage axis of the considered deflector,

each of said deflectors having first and second conjugate real stigmaticpoints respectively lying on said object axis and said image axis of theconsidered deflector, said first deflector being located to have itsobject axis along said axis of said first lens means, and its firststigmatic point at said predetermined point of said axis of said lensmeans, 7

said second deflector being located to have its object axis along theimage axis of said first deflector, the superimposed last two-mentionedaxes being referred to as the common axis of said filtering means,

said filtering means further comprising a diaphragm centered on saidcommon axis at said second conjugate stigmatic point of said firstdeflector and second lens means, having an axis coinciding with saidcommon axis, for forming in an image plane a further image of the imageformed by said first deflector, at least as far as radial focusing isconcerned, of said first image of said surface and for forming, at saidfirst conjugate real stigmatic point of said second deflector, an imageof said second conjugate real stigmatic point of said first deflector;

a screen,

and further lens means having an axis coinciding with said image axis ofsaid second deflector, for projecting said selective ionic image ontosaid screen;

said further image in said image plane being inverted or erect accordingto whether said first lens means and said further lens means lie on oneand the same side or on opposite sides of that plane, normal to saidradial plane, which passes through said axis of said second lens means.

2. An ionic microanalyzer as claimed in claim 1 wherein,

each of said deflectors having for said optical axis thereof anachromatic focal point located on the image axis thereof,

and said further point of said axis of said first lens means coincidingwith the conjugate relatively to said first deflector, at least as faras radial focusing is concerned, of said achromatic focal point of saidfirst deflector,

said second lens means being further such that said image planeintersects said axis of said second lens means at a point which, inrelation to the second deflector, is the conjugate, at least as far asradial focusing is concerned, of said achromatic focal point of saidsecond deflector.

3. An ionic microanalyzer as claimed in claim 2, wherein said first lensmeans and said further lens means lie on one and the same side of saidplane normal to said radial plane and wherein said second lens meanscomprise a single lens.

4. An ionic microanalyzer as claimed in claim 2, wherein said first lensmeans and said further lens means respectively lie on opposite sides ofsaid plane normal to said radial plane and wherein said second lensmeans comprise a first and a second lens.

5. An ionic microanalyzer as claimed in claim 1, wherein: said axis ofsaid first lens means and said axis of said further lens means areparallel to each other and perpendicular to said axis of said secondlens means; said axis of said second lens means and that one of the twoaxes of said first and further lens means which is intersected by one ofsaid lateral faces of said gap make angles equal in absolute value toArc tan is with the normals at said lateral faces of said gap at theirrespective points of intersection with said lateral faces; and astigmator is provided for correcting the astigmatism of the image ofsaid surface provided by said magnetic deflector.

6. An ionic microanalyzer as claimed in claim 5, wherein said diaphragmis centered on said axis of said second lens means at a point located atthe distance D=2Rr from the point at which said last-mentioned axis istangent to said are of a circle of said optical axis of said sphericalcapacitor, R and r being respectively the radii of said arcs of a circleof said optical axes of said magnetic deflector and of saidelectrostatic deflector respectively.

2. An ionic microanalyzer as claimed in claim 1 wherein, each of saiddeflectors having for said optical axis thereof an achromatic focalpoint located on the image axis thereof, and said further point of saidaxis of said first lens means coinciding with the conjugate relativelyto said first deflector, at least as far as radial focusing isconcerned, of said achromatic focal point of said first deflector, saidsecond lens means being further such that said image plane intersectssaid axis of said second lens means at a point which, in relation to thesecond deflector, is the conjugate, at least as far as radial focusingis concerned, of said achromatic focal point of said second deflector.3. An ionic microanalyzer as claimed in claim 2, wherein said first lensmeans and said further lens means lie on one and the same side of saidplane normal to said radial plane and wherein said second lens meanscomprise a single lens.
 4. An ionic microanalyzer as claimed in claim 2,wherein said first lens means and said further lens means respectivelylie on opposite sides of said plane normal to said radial plane andwherein said second lens means comprise a first and a second lens.
 5. Anionic microanalyzer as claimed in claim 1, wherein: said axis of saidfirst lens means and said axis of said further lens means are parallelto each other and perpendicular to said axis of said second lens means;said axis of said second lens means and that one of the two axes of saidfirst and further lens means which is intersected by one of said lateralfaces of said gap make angles equal in absolute value to Arc tan 1/2with the normals at said lateral faces of said gap at their respectivepoints of intersection with said lateral faces; and a stigmator isprovided for correcting the astigmatism of the image of said surfaceprovided by said magnetic deflector.
 6. An ionic microanalyzer asclaimed in claim 5, wherein said diaphragm is centered on said axis ofsaid second lens means at a point located at the distance D 2R-r fromthe point at which said last-mentioned axis is tangent to said arc of acircle of said optical axis of said spherical capacitor, R and r beingrespectively the radii of said arcs of a circle of said optical axes ofsaid magnetic deflector and of said electrostatic deflectorrespectively.